Fix #20: handle activity task without input in ActivityTask.from_poll()
Fix #21: support region in swf.settings

- rename swf.credentials in swf.settings
- rename functions in swf.settings
- use connect_to_region() in swf.core.ConnectedSWFObject
- update docs/quickstart
- add docs/api/settings
Fix tests: replace set_aws_credentials() by swf.settings.set()
Fix ConnectedSWFObject: pass default value to pop()
Fix ConnectedSWFObject: restrict attributes set by constructor

- credentials: SETTINGS | kwargs
- region: SETTINGS | kwargs | boto.swf.layer1.Layer1.DefaultRegionName
- connection: kwargs
Fix ModelDiff: add missing operators and instanciate ModelDiff.container as an OrderedDict()

- Add:
  - __len__
  - __getitem__: returns a Difference(attr, local, upstream) object
- ModelDiff.container is now an OrderedDict instead of a dict
- fix tests:
  - compare len() of empty ModelDiff instead of []
  - use right attribute names for Difference objects
Fix tests.models.test_event: fix calls to constructor and len()
Merge master into events_and_actors
Merge branch 'events_and_actors'